1 .. _debugging-jited-code:
3 ==============================
4 Debugging JIT-ed Code With GDB
5 ==============================
7 .. sectionauthor:: Reid Kleckner and Eli Bendersky
12 Without special runtime support, debugging dynamically generated code with
13 GDB (as well as most debuggers) can be quite painful. Debuggers generally
14 read debug information from the object file of the code, but for JITed
15 code, there is no such file to look for.
17 In order to communicate the necessary debug info to GDB, an interface for
18 registering JITed code with debuggers has been designed and implemented for
19 GDB and LLVM MCJIT. At a high level, whenever MCJIT generates new machine code,
20 it does so in an in-memory object file that contains the debug information in
21 DWARF format. MCJIT then adds this in-memory object file to a global list of
22 dynamically generated object files and calls a special function
23 (``__jit_debug_register_code``) marked noinline that GDB knows about. When
24 GDB attaches to a process, it puts a breakpoint in this function and loads all
25 of the object files in the global list. When MCJIT calls the registration
26 function, GDB catches the breakpoint signal, loads the new object file from
27 the inferior's memory, and resumes the execution. In this way, GDB can get the
28 necessary debug information.
33 In order to debug code JIT-ed by LLVM, you need GDB 7.0 or newer, which is
34 available on most modern distributions of Linux. The version of GDB that
35 Apple ships with Xcode has been frozen at 6.3 for a while. LLDB may be a
36 better option for debugging JIT-ed code on Mac OS X.
39 Debugging MCJIT-ed code
40 =======================
42 The emerging MCJIT component of LLVM allows full debugging of JIT-ed code with
43 GDB. This is due to MCJIT's ability to use the MC emitter to provide full
44 DWARF debugging information to GDB.
46 Note that lli has to be passed the ``-use-mcjit`` flag to JIT the code with
47 MCJIT instead of the old JIT.
52 Consider the following C code (with line numbers added to make the example
57 Sphinx has the ability to automatically number these lines by adding
58 :linenos: on the line immediately following the `.. code-block:: c`, but
59 it looks like garbage; the line numbers don't even line up with the
60 lines. Is this a Sphinx bug, or is it a CSS problem?
64 1 int compute_factorial(int n)
76 13 int main(int argc, char** argv)
80 17 char firstletter = argv[1][0];
81 18 int result = compute_factorial(firstletter - '0');
83 20 // Returned result is clipped at 255...
87 Here is a sample command line session that shows how to build and run this
88 code via ``lli`` inside GDB:
92 $ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
93 $ gdb --quiet --args $BINPATH/lli -use-mcjit showdebug.ll 5
94 Reading symbols from $BINPATH/lli...done.
96 No source file named showdebug.c.
97 Make breakpoint pending on future shared library load? (y or [n]) y
98 Breakpoint 1 (showdebug.c:6) pending.
100 Starting program: $BINPATH/lli -use-mcjit showdebug.ll 5
101 [Thread debugging using libthread_db enabled]
103 Breakpoint 1, compute_factorial (n=5) at showdebug.c:6
113 (gdb) b showdebug.c:9
114 Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9.
118 Breakpoint 2, compute_factorial (n=1) at showdebug.c:9
123 #0 compute_factorial (n=1) at showdebug.c:9
124 #1 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
125 #2 0x3500000001652748 in ?? ()
126 #3 0x00000000016677e0 in ?? ()
127 #4 0x0000000000000002 in ?? ()
128 #5 0x0000000000d953b3 in llvm::MCJIT::runFunction (this=0x16151f0, F=0x1603020, ArgValues=...) at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/MCJIT/MCJIT.cpp:161
129 #6 0x0000000000dc8872 in llvm::ExecutionEngine::runFunctionAsMain (this=0x16151f0, Fn=0x1603020, argv=..., envp=0x7fffffffe040)
130 at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/ExecutionEngine.cpp:397
131 #7 0x000000000059c583 in main (argc=4, argv=0x7fffffffe018, envp=0x7fffffffe040) at /home/ebenders_test/llvm_svn_rw/tools/lli/lli.cpp:324
133 Run till exit from #0 compute_factorial (n=1) at showdebug.c:9
134 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
135 18 int result = compute_factorial(firstletter - '0');
136 Value returned is $5 = 120
146 Program exited with code 0170.